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# V21-07

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Manager
Joined: 19 May 2015
Posts: 128

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15 Feb 2018, 19:33
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45% (medium)

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50% (01:17) correct 50% (00:42) wrong based on 10 sessions

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The popular belief that a snake's age can be determined by counting the number of layers of scales on its body is generally true. However, to help regulate its internal temperature, the black mamba snake often sheds its outermost layers of scales when the temperature exceeds approximately 120 degrees Fahrenheit, leaving the snake with fewer layers of scales than it would otherwise have. Thus, if a black mamba snake has frequently been exposed to temperatures exceeding 120 degrees Fahrenheit, counting the number of layers of scales on its body will most likely result in an inaccurate calculation of the snake's age.

Which of the following is an assumption on which the argument is based?

A. A black mamba snake must be exposed to temperatures above 120 degrees Fahrenheit for several hours before shedding its outermost layers of scales.
B. A black mamba snake will not consistently regenerate the layers of scales it sheds after exposure to high temperatures.
C. All black mamba snakes that have frequently been exposed to temperatures exceeding 120 degrees Fahrenheit have shed at least one layer of scales.
D. Exposure to temperatures exceeding approximately 120 degrees Fahrenheit is the only environmental factor that triggers black mamba snake to shed its outermost layers of scales.
E. The black mamba snake grows no more than two new layers of scales on its body each year.
Manager
Joined: 19 May 2015
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15 Feb 2018, 19:33
Official Solution:

The popular belief that a snake's age can be determined by counting the number of layers of scales on its body is generally true. However, to help regulate its internal temperature, the black mamba snake often sheds its outermost layers of scales when the temperature exceeds approximately 120 degrees Fahrenheit, leaving the snake with fewer layers of scales than it would otherwise have. Thus, if a black mamba snake has frequently been exposed to temperatures exceeding 120 degrees Fahrenheit, counting the number of layers of scales on its body will most likely result in an inaccurate calculation of the snake's age.

Which of the following is an assumption on which the argument is based?

A. A black mamba snake must be exposed to temperatures above 120 degrees Fahrenheit for several hours before shedding its outermost layers of scales.
B. A black mamba snake will not consistently regenerate the layers of scales it sheds after exposure to high temperatures.
C. All black mamba snakes that have frequently been exposed to temperatures exceeding 120 degrees Fahrenheit have shed at least one layer of scales.
D. Exposure to temperatures exceeding approximately 120 degrees Fahrenheit is the only environmental factor that triggers black mamba snake to shed its outermost layers of scales.
E. The black mamba snake grows no more than two new layers of scales on its body each year.

On strengthen, weaken, or assumption questions, I always like to start with a nice, clear restatement of the conclusion:

"...if a black mamba snake has frequently been exposed to temperatures exceeding 120 degrees Fahrenheit, counting the number of layers of scales on its body will most likely result in an inaccurate calculation of the snake's age."

Great. And what's the reasoning the author used to reach this conclusion? We know the following:

Generally, a snake's age can be determined by counting the number of layers of scales on its body. The black mamba snake often sheds its outermost layers of scales when the temperature exceeds approximately 120 degrees Fahrenheit, leaving the snake with fewer layers of scales than it would otherwise have.

So apparently, the black mamba grows layers of scales at a constant rate as it ages, but will lose some of those layers if it's exposed to high temperatures. And we're looking for something that will reinforce the conclusion that "counting the number of layers of scales on its body will most likely result in an inaccurate calculation of the snake's age."

A) A black mamba snake must be exposed to temperatures above 120 degrees Fahrenheit for several hours before shedding its outermost layers of scales.

The author's argument is based on the premise that "to help regulate its internal temperature, the black mamba snake often sheds its outermost layers of scales when the temperature exceeds approximately 120 degrees Fahrenheit." At best, (A) is irrelevant, because it wouldn't do anything to impact whether we can accurately determine the snake's age.

But you could also argue that statement (A) would actually weaken the author's argument. For example, consider the black mamba snake that has been exposed to temperatures exceeding 120 degrees Fahrenheit four times in its life. The author would expect that this snake lost its outermost layer of scales each of those four times. But what if the snake was only exposed to those high temperatures for an hour each time? If statement (A) were true, that snake would not have shed its outermost layers of scales, and counting the number of layers of scales would likely result in an accurate calculation of the snake's age.

Either way, choice (A) can be eliminated.

B) A black mamba snake will not consistently regenerate the layers of scales it sheds after exposure to high temperatures.

If a black mamba consistently regenerates the layers of scales that it sheds after exposure to high temperatures, then exposure to high temperatures would have no effect on the number of layers of scales that the snake has. For example, a five-year-old black mamba might normally have 10 layers of scales. If that black mamba lost four layers of scales because of exposure to high temperatures, the author would expect that snake to only have six layers of scales. But if the snake regenerated each layer of scales that it lost, then the snake would indeed have 10 layers and counting the number of layers of scales would likely result in an accurate calculation of the snake's age.

Thus, the author's argument relies on the fact that the black mamba will NOT consistently regenerate the scales it sheds after exposure to high temperatures. Choice (B) is therefore a necessary assumption, and it looks pretty good.

C) All black mamba snakes that have frequently been exposed to temperatures exceeding 120 degrees Fahrenheit have shed at least one layer of scales.

The author does not claim that exposure to high temperatures causes ALL black mamba snakes to shed at least one layer of scales. Rather, the author states that "the black mamba snake often sheds its outermost layers of scales when the temperature exceeds approximately 120 degrees Fahrenheit" (often, not always). Thus, the author's argument does not rely on the assumption stated in choice (C).

D) Exposure to temperatures exceeding approximately 120 degrees Fahrenheit is the only environmental factor that triggers black mamba snake to shed its outermost layers of scales.

Because "the black mamba snake often sheds its outermost layers of scales when the temperature exceeds approximately 120 degrees Fahrenheit", the author concludes that "if a black mamba snake has frequently been exposed to temperatures exceeding 120 degrees Fahrenheit, counting the number of layers of scales on its body will most likely result in an inaccurate calculation of the snake's age." Even if other environmental factors also trigger black mamba snakes to shed their outermost layers of scales, the author's argument, which is only concerned with exposure to temperatures exceeding 120 degrees Fahrenheit, would not be impacted.

However, consider the following alternative conclusion: "if a black mamba snake has NOT frequently been exposed to temperatures exceeding 120 degrees Fahrenheit, then counting the number of layers of scales on its body will most likely result in an ACCURATE calculation of the snake's age." If the author had reached that conclusion instead, then the author's argument would have required making the assumption stated in choice (D). However, as is, the author's argument does not rely on the assumption stated in choice (D).

E) The black mamba snake grows no more than two new layers of scales on its body each year.

It doesn't matter how many layers of scales a black mamba grows each year. As long as the number of layers grown per year is known, one could theoretically calculate the snake's age by counting the number of layers. Choice (E) is not a required assumption.

Choice (B) is the best answer.

Manager
Joined: 21 Jul 2014
Posts: 66
GMAT Date: 07-30-2015

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21 Mar 2018, 01:50
I Chose Option D on the basis of cause and effect relation. ie.It's only to exposure to 120 degree heat that causes Black Mamba to shed it's layer's.
120 degrees Fahrenheit is the only environmental factor that triggers black mamba snake to shed its outermost layers of scales.

If we negate doesn't the conclusion fall apart. Doesn't this option needs to be true for the conclusion to follow, Please explain?
Re: V21-07 &nbs [#permalink] 21 Mar 2018, 01:50
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# V21-07

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